Animal behaviour (II)

Sexual appetite

Being a male spider is even more dangerous than previously thought

FOR male spiders, the equivalent of a post-coital cigarette is not a sensible option. Making straight for the door is much wiser, since he who hangs around risks being eaten by his consort. In this context, the females of Leucauge argyra, a Central American species, are particularly fiendish. They seem to have subverted a male trick that is intended to ensure paternity rights, and turned it into a trap.

Many animals—not just spiders—make plugs of mucus to seal the female's orifice after mating. Normally such plugs are secreted by the male, to stymie subsequent suitors. In spiders, though, the female sometimes assists the process.

At first sight, that is odd. It is always in a male's interest to try to stop a female mating again. Contrariwise, that female might come across a better male and thus wish to be rid of the first lot of sperm. (Female spiders can do this because they store sperm for a period of days or weeks before they use it to fertilise their eggs.) After studying this phenomenon for several years, Anita Aisenberg of the Clemente Estable Institute of Biological Research in Uruguay has concluded that females are able to assess a male's quality during mating and that they assist plug formation only when a mate passes muster and they thus wish to give his sperm priority. Which makes sense. In a paper just published in Naturwissenschaften, though, Dr Aisenberg and her colleague Gilbert Barrantes at the University of Costa Rica report that female Leucauge argyra go further: they create plugs all by themselves.

And that really is odd. Because plugging is in a male's interest, he should never leave it just to the female. Except that in this case the plug turns out to be very much not in a male's interest. In fact, it can kill him.

Spider sex is unusual in that males transfer their sperm to the female on small limbs called pedipalps. They use these to pick their sperm up from their genitals and insert it into the female's sexual orifice, rather than copulating directly. Dr Aisenberg and Dr Barrantes observed the process 17 times in pairs of Leucauge argyra. On the 14 occasions a sexual plug was made, the female produced it without assistance from the male. On ten of these occasions the male's pedipalps then seemed to get stuck while he was transferring the sperm (which is rarely the case in other species of spider), and he had great difficulty freeing himself. In two of those ten instances, he was eaten as a result.

The two researchers conclude, therefore, that what was once a mechanism which allowed females to discriminate between males has evolved into a way of trapping them so that they can be consumed at leisure. Whether only high-quality males, whose sperm the female also wishes to keep, are affected is not yet clear—though eating such males may bring the bonus of denying their sperm to rivals. Whatever the details, for male Leucauge argyra this behaviour gives a new meaning to the term “unsafe sex”.